Having fun tonight with an IBM 5150 with 64-256 KB board and apparently fault memory. The error is 406E 201, a 64-256 board and no ram expansion cards. The switches for the memory are on-off-off-on-on and the motherboard has 4 banks of 64 KB.
The "406E 201" RAM error produced by your 64KB-256KB 5150 motherboard equates to:
Address: 256K
Bits in error: 1,2,3,5,6
That can be determined by the information at:
1. Goto
www.minuszerodegrees.net
2. Choose 'IBM 5150 - RAM Information'
3. Choose '64KB-256KB Motherboard'
So, for whatever reason, your 64KB-256KB 5150 motherboard (with no RAM cards fitted) believes that more than 256K is present and has attempted to test RAM that doesn't exist.
For your 64KB-256KB 5150 motherboard (with no RAM cards fitted), the motherboard switches should be:
SW1: Switches 3 and 4 both off.
SW2: 1=on, 2=off, 3=off, 4=on, 5=on
The switches for the memory are on-off-off-on-on
That is right for SW2, but the symptoms suggest a problem with SW2, either incorrect settings, faulty switch within SW2, or a switch not quite positioned 100%.
Try switching each of the SW2 switches to the alternate position and then back.
Just in case, also take a look at
http://www.minuszerodegrees.net/5150_5160/misc/5150_5160_on_versus_off.htm